Field of the Invention
The present invention relates to an image sensing device, and a control method thereof.
Description of the Related Art
In recent years, advancement has been made in functionality and versatility of image sensing devices that use a CMOS image sensor and the like in order to cater to various types of needs. Along with an increase in the number of pixels and the speed of imaging, there is a growing need for a type of CMOS image sensor that can read out pixel signals at a higher speed. In order to achieve high speed readout, a method is widely used in recent years in which an analog-to-digital conversion circuit (hereinafter referred to as a column ADC) is disposed for each column so as to provide a digital output (Japanese Patent Laid-Open No. 2005-278135). The use of column ADCs allows the image sensor to digitally transmit pixel signals to the outside, and with an improvement in the digital signal transmission technique, high speed readout has become possible.
In Japanese Patent Laid-Open No. 2005-278135, a single slope method is used as a column AD conversion method. The single slope method is a method that uses a comparator and a counter and in which while an analog pixel signal is input into one of the input terminals of the comparator, a slope voltage is input into the other input terminal as a reference signal, the slope voltage being a voltage that varies with inclination along with time. The output of the comparator is inverted at the time at which the magnitude relationship between the pixel signal and the reference signal is inverted. The counter digitally counts up with time, and stops counting when the output of the comparator is inverted. The output of the counter when counting is stopped represents the digital value of the analog pixel signal. AD conversion is thereby performed. In the case of the single slope method, in order to achieve a high bit resolution signal output, the counting time of the counter increases. For example, in order to increase the resolution from 12 bits to 14 bits, a four times longer time is required as the AD conversion period if the counter has a constant operation clock.
Also, a technique is proposed in Japanese Patent Laid-Open No. 2013-9087 that enables higher speed readout by switching between a low bit resolution ADC and a high bit resolution ADC according to the signal output. Considering light shot noise characteristics of pixel signals, because a large amplitude signal only needs to have a resolution enough to ensure a SN ratio, in the case where the select circuit included in the ADC performs AD conversion on a large amplitude signal, a slope voltage having a large inclination is selected to perform low bit resolution AD conversion. On the other hand, in the case of AD conversion of a small amplitude signal, a slope voltage having a small inclination is selected to perform high bit resolution AD conversion. By doing so, the time required to perform AD conversion of a large amplitude signal, which takes a long time in conventional AD conversion, can be shortened although the AD conversion is low bit resolution AD conversion.
It is known that a dark current is generated in an image sensor when exposure is performed for a long period of time or when image capturing is performed under a high temperature environment. The dark current is proportional to exposure time, and therefore under an imaging condition in which exposure is performed for a long period of time, the output caused by a dark current increases, and its influence cannot be ignored. The dark current is detrimental to image quality, and thus processing is performed to subtract a dark current component from an optical signal component, but the processing is usually performed after the output has been digitalized.
In the case of the method as disclosed in Japanese Patent Laid-Open No. 2013-9087 in which low bit resolution AD conversion is performed when AD conversion is performed on a large amplitude signal, image quality is deteriorated under an imaging condition in which the dark current is large. This is because the amplitude increases due to the dark current even when the actual optical signal has a small amplitude, and the optical signal having a small amplitude is AD converted with a low bit resolution.